Oxytocin, the peptide hormone produced in the hypothalamus, increases the contractility of its physiological target tissues, the uterine smooth muscle and myoepithelial cells of the breast. The molecular mechanism by which oxytocin exerts its effect is still unknown, mainly because of the great experimental difficulty of studying these tissues. A promising alternative for such studies appears to be the Xenopus oocyte. Preliminary experiments have shown that oocytes acquire oxytocin sensitivity when injected with a mRNA preparation derived from estrogen-primed uterine smooth muscle. Estrogen treatment is known to induce the same changes in the myometrium as occuring in preparation for parturition: upregulation of oxytocin receptors, gap junctions, and of Alpha-adrenergic receptors. In parallel to acquisition of oxytocin sensitivity, pairs of mRNA-injected oocytes also develop gap junctions. The ocytocin response in mRNA-injected oocytes consists of a dose-dependent, sustained and reversible depolarization of the cell membrane. This hormone-induced change in membrane potential indicates that oxytocin receptors are incorporated into the oocyte membrane and are either linked to ion channels or are ion channels on their own. Xenopus oocytes are readily amenable to electrophysiological studies and it is proposed to use the mRNA induction of oxytocin sensitivity in these cells to identify the charge carrying ion species and test criteria for second messengers. The studies will be started with whole cell voltage clamp techniques to characterize the oxytocin response at the macroscopic level. Subsequently patch clamp methods will be employed to continue the investigation at the single channel level.